1. Field of the Invention
The invention relates to a mowing device for crops, and more particularly to a mowing device with a series of knife holders with mowing knives rotated by a drive mechanism coupled to a power take-off or another external drive device.
2. Description of the Related Art
The invention relates to a mowing device for crops, such as grass, comprising a series of knife holders with mowing knives, arranged side by side, which knife holders are rotated about vertical center lines by means of a drive mechanism coupled to a power take-off or another external drive device. In this case, the knife holders are fastened in a rotationally fixed manner on associated hubs which are driven by the drive mechanism.
An example of such a mowing device is disclosed in patent application EP A 0,126,518. In this case, the drive mechanism comprises a drive shaft which extends in the direction of the series of knife holders. For the transmission of the drive shaft to the knife holder there is each time used a case which is provided at one end with a conical gearwheel and which is fastened on the drive shaft. Another conical gearwheel is in engagement with the former gearwheel and has a vertical rotational center line or axis. The other gearwheel forms, together with a hub, a rotatable unit which is bearing-supported in a housing and which supports and rotates the knife holder. In the known device, the knife holder and the hub are interconnected by means of a hood and a single, central nut connection.
In later mowing devices, the knife holder is fastened on a hub flange by means of a plurality of bolts, usually by means of four bolts which are regularly spaced apart in a circumferential direction and which extend through bolt holes into the knife holder and the hub flange. The bolt heads may be protected by means of a hood which is secured on the hub flange by means of a small bolt.
Upon mowing grass mechanically, it is possible to encounter whether or not foreign bodies, such as stones, steel wires, cables, tubes, but also big objects which were left on the spot as waste. This may especially occur in roadsides.
When such a hard, foreign body is hit by mowing knives, overload of the drive mechanism occurs. Replacement of components of the central drive mechanism leads to inadmissible stagnation, and to replacement of less accessible components of the drive elements.
U.S. Pat. No. 4,497,161 discloses a mowing device in which the knife holder is fastened, by means of vertical breaking bolts, on a hub flange which is connected to a central, vertical axis which is rotated. The breaking bolts are upwardly retained by a hood which is fastened on the axis by means of a central bolt. In an alternative embodiment, the hood is resilient and, instead of breaking bolts, there are disposed, between the hub flange and the knife holder, a number of balls which are received in holes in the knife holder and extend into cavities in the hub flange. In the case of an impact of the knives, it will be possible for the balls to move upwardly from the cavities, in which case the hood will deform as well.
Patent application EP 0,774,202 discloses a mowing device in which the knife holder is fastened by means of bolts to an annular hub flange which, by means of a ring that is disposed at the inner circumference of the hub flange, is connected to a vertical axis which is provided at its lower end with a to be driven gearwheel. The ring is axially kept in place by means of a central bolt and a hood. In an embodiment, the ring has inner protrusions which break in the case of an impact, after which the axis does no longer drive the hub flange positively into rotation. To replace the ring the hood may be removed, but, for practical reasons, the hub together with the knife holder will be removed as well, in order to be able to mount the new ring in an easy manner (first ring around vertical axis and then hub around ring) and to be able to remove broken teeth. In this case, there is a risk of penetration of dirt into the bearing.
U.S. Pat. No. 6,675,563 discloses a mowing device which comprises an axially divided hub flange, in which case a knife holder is fastened on the upper hub flange portion by means of bolts and the upper flange portion and the lower flange portion are interconnected by means of a spring loaded ball. In the case of an impact, it is possible for the ball to move axially against spring force in order to enable mutual rotation of both hub flange portions. In this case, the upper hub flange portion will move upwardly as a result of engagement with a central screw thread.
Patent application EP 0,366,580 discloses a mowing device in which the knife holder is fastened on a hub flange by means of bolts, and the hub flange is fixed to a vertical, rotating axis which is provided with a circumferential groove so as to form a weakening. In the case of an impact, the axis will break and the upper part of the axis with the hub flange will come free.